Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Elliptical reflector

Demonstration

A circular pulse started at one focus will be reflected as a straight pulse from the nearest part of the elliptical barrier. The straight pulse will then travel to the farthest part of the barrier to be reflected as a circular pulse centred on the second focus.

Apparatus and materials

Ripple tank and accessories

Water dropper

Elliptical reflector

Health & Safety and Technical notes


Beware of water on the laboratory floor. Make sure you have a sponge and bucket handy to mop up spills immediately. 
 
Place the power supply for the lamp on a bench, not on the floor by the tank.

1 The beauty of this demonstration is very sensitive to the accuracy of the ellipse. Some manufacturers supply such an elliptical reflector in the ripple tank kit as an optional extra. 

To be sure of the necessary accuracy, you may prefer to make your own barrier. First draw an ellipse very carefully on paper. Someone skilful can then bend a springy brass strip to fit the ellipse, joining the ends with a butt joint and a strap outside. (This type needs very careful storage.) 
 
Alternatively, you can draw the ellipse on plywood and then cut it out to the required shape. Great care must be taken if this method is used. 
 
To make the best reflector of all, use a wall of plaster of Paris, drawing the ellipse with a peg moving along a loop of wire. 
 
2 Coating the reflector with paraffin wax may improve the regularity of reflection by making an angle of contact with the walls exactly 90° 

 

Procedure


Place the elliptical reflector in the middle of a ripple tank with very clean water. Start a single ripple accurately at one focus and watch its progress. 

The position of one focus of the ellipse must be located very accurately and used as the starting point of the ripples. Perhaps the best way of finding the focus is 'trial by ripples'. Then place two small coins in the tank to mark the two focuses. 


Teaching notes


This is an intriguing ripple tank experiment to watch. Image formation by a wide-aperture reflector like this depends on the wave path being the same from object to image by all routes, even those that use extreme portions of the aperture. An ellipse does this - though it fails to give a good image of points a little way off the focus. 

However, if part of the reflecting surface is a little off the true ellipse, the condition fails and reflection there may even harm the image instead of helping to form it. An error of 1/4 wavelength in part of the surface will do great harm. Considering how small the actual wavelength of the equivalent ripples in a pulse must be, this error is very small. 

This experiment was safety-checked in February 2006